This invention is a continuation-in-part of U.S. Ser. No. 193,482, filed Oct. 28, 1971 and relates to xerography and more specifically to a method of treating photosensitive zinc oxide in order to enhance its development characteristics.
In the art of xerography, a photosensitive member comprising a binder layer, such as zinc oxide particles dispersed in a film forming insulating resin, is uniformly electrostatically charged in the dark and then exposed to pattern of activating radiation to form a latent electrostatic image on the surface of the binder layer. This latent image may then be developed by immersing the photosensitive member in a liquid developing solution which contains toner particles. The toner particles are attracted to and adhere to the areas containing the latent electrostatic image. After removal from the developer bath, the photosensitive member is dried and the toner image fused to form a permanent reproduction of the original radiation pattern or image.
It is well known in the art of xerography that it is difficult to adjust the properties of a photosensitive layer to render it suitable for continuous tone reproduction when a photosensitive layer is imaged in the conventional sequence of charging, exposure and development with toner. In the art of photography, there are available photographic papers having grades designated No. 1 to No. 5 of silver salt having the characteristic curves for soft to hard tones. In electrophotography, however, the conventional photoconductor, such as photosensitive zinc oxide contained in a film forming insulating resin, is characterized by straight line in that the characteristic curve is shorter and the incline of the curve usually larger, which means harder. This characteristic curve is prepared by plotting the logarithm of the exposure strength on the abscissa and the relative residual potential or developer concentration at the ordinate. In general, no appreciable effect on the adjustment of the grading of the characteristic curve can be obtained in zinc oxide binder systems by changing such processing parameters as kneeding conditions, powder sizes, or the ratio of the resin to the photoconductor component.
To some extent the art has resorted to adjusting a wide range of such properties by utilizing two different kinds of zinc oxides having different light sensitivity. One kind of zinc oxide is used as a continuous layer and the other allowed to be distributed over said layer in a spotted manner to create a light sensitive layer. This concept is more fully described in U. S. Pat. No. 3,003,870. Similarly, the art has also resorted to using laminated coatings of several light sensitive component layers, one upon the other, of different spectrum sensitivity in order to otain enhanced electrical response. This concept is more fully described in British Patent Specification 967,690.
Notwithstanding the above, these techniques, suggested by the prior art, involve the necessity of coating a plurality of times in order to attempt to improve the electrical characteristics of zinc oxide binder systems.
It can be seen from the prior art, that complicated manufacturing processes, requiring much labor and resulting in lower efficiency in production, are required in order to attempt to obtain photoreceptors exhibiting a soft characteristic suitable for continuous tone reproduction. Accordingly, the use of the conventional techniques described above still render it difficult to prepare photoreceptor, such as the zinc oxide binder type, which exhibits tone characteristics soft enough to meet the requirements for continuous tone reproduction.
In using a photosensitive member which comprises zinc oxide contained in a film forming insulating resin, it is well known that zinc oxide must be used only with negative charging in that it has poor acceptance for positive charge. Accordingly, such a photosensitive layer must use a converted developing polarity in changing an original image from a positive to negative because of such a limitation. It is therefore required to employ during the developing method, a developer toner which floats electrically into the electrostatically latent part of the image for positive development, and then is developed (reversal development) by depositing toner provided with the same polarity of that of the latent electrostatic image. It is necessary in positive development of zinc binder layers to use a development liquid having a positive polarity, and for negative development (reversal development) to use a development liquid having a negative polarity toner. A problem associated with this requirement for both positive and negative images is that a toner mix be provided which has different polarities and which must all be incorporated into the same developing solution. This results in many instances in a deterioration of the image quality resulting from a required mix of such toner. In addition, it is very difficult to get stable developing liquid having a negative polarity toner. This is because of the fact that most colored pigments used for toner such as carbon black and phthalocyanine pigment have a positive polarity.
In addition, most resins which provide negative polarity toner are inconvenient with respect to their inability to properly moisten, especially to obtain a stable dispersive liquid for developing. On the other hand, developing liquid with a positive polarity toner is more easily obtainable. These include, for example, alkyd resins, resin-modified formaldehyde, and in addition, provide good moistening of the pigment and are easily dissolved in a developing liquid.
For the above mentioned reasons, it is desirable that an electrophotosensitive layer be capable of accepting charges of both positive and negative polarity.